in recent years, in the field of sensors and actuators, it has been attempted to manufacture new devices, such as infrared sensors using abridged structure (thermobolometers), high-sensitivity capacitive pressure sensors using a very thin diaphragm, humidity sensors and audible range-controllable acoustic elements, by forming precision mechanical structures having a size of several microns on a substrate made of a semiconductor material, such as silicon.
Conventionally, in a case where a diaphragm structure of silicon having a smaller thickness than several microns is formed using silicon as a material, etching processing has been performed using alkaline etchant for example. In this case, it is important to control the stop timing of etching which influences the accuracy in the etching processing. As methods for stopping the etching, so called p-n etch-stop technique using p-n junction bias, and etch-stop technique using high-density impurity diffusion have been used ("TRANSDUCERS '87" pages 116-125).
however, in a case where a thin diaphragm is formed by the etch-stop technique using the p-n junction bias, an expensive epitaxial wafer is needed. Furthermore, this technique requires to establish an electrical contact with the p-type part and the n-type part of a silicon wafer immersed in an electrolytic solution, and to form these electrodes insulated from the electrolytic solution. However, it is technically difficult to satisfy these requirements. In the case of the etch-stop technique using the high-density impurity diffusion, it is difficult to selectively diffuse impurities, and heat treatment must be performed at a high temperature for a long time. In addition, it is difficult to control the concentration of impurity in the direction of depth. Moreover, since this technique can not cause the etching to be completely stopped, it is difficult to form a desired structure with high accuracy.
It has been known to produce a diaphragm or a bridged structure by laminating a silicon oxide film (SiOx) and a silicon nitride film (SiNy) on a silicon substrate and by etching apart of the silicon substrate. In this method, it is required to accurately control the conditions for the formations of the silicon oxide film and the silicon nitride film so as to ensure the counter balance between the elongation stress of the silicon oxide film and the contraction stress of the silicon nitride film. However, this control is very difficult in realization, and the product tends to be broken due to unbalance between the stresses applied to the respective films. Therefore, it is difficult to improve the yield of the diaphragm or the bridged structure.